Adaptation of the SGT6-6000G to a Dynamic Power Generation Market

This paper describes the U.S. market conditions in which the SGT6-6000G was designed and its evolution to accommodate the current and future market requirements. The U.S. market drivers during the SGT6-6000G design were deregulation and replacing the old base load generation, such as old coal plants, with high efficiency/output power and low emissions combined cycle plants. The SGT6-6000G was originally designed primarily for base load operation, but also with sufficient cyclic life margin on its critical components to make it inherently suitable for cyclic operation. The current natural gas price and the power generation overcapacity created by the gas turbine boom have resulted in a competitive situation which places a premium on minimized operational costs and maximized flexibility. This phenomenon resulted in a situation where usually the nuclear and coal plants are dispatched first and only then the most economical and operationally flexible gas turbine plants would be dispatched. Instead of operating full time at base load, SGT6-6000G is now required to operate predominantly in the cyclic mode. To address the market shifts and customer requirements, improvements were made to the gas turbine and the plant to enhance its capability for low cost, flexible operation. These improvements were focused on increased cycling capabilities, increased reliability, improved part load and base load performance, reduced emissions (especially at part load), increased time between inspections and reduced plant startup times. Introduction The market drivers during the SGT6-6000G gas turbine design (1993-1995) were moving toward deregulation and replacing old base load generation, such as old coal-based power plants, with clean and efficient combined cycle plants. Fears of deregulation in the North American electricity market caused prospective plant buyers to look at plants that could be installed and commissioned in a short time, rather than the 6 years required to permit and build a coal plant. The belief at the time was that the new high efficiency, low emissions, clean fuel plants would be the economic and environmental choice and displace coal plants. Because of its high efficiency (58% in CC application compared to the then state-of-the-art CC plants with 54-55% efficiencies), it was intended that the SGT6-6000G would be operated primarily at base load. In the late 1990's there was an increased demand for electric power. Since natural gas prices were low (about $2.50/MMBtu) and there was a large demand for increased power generation, many gas turbines were bought for simple and combined cycle operation. By 2002, the demand for power was subsiding (in some areas there was overcapacity), and the availability and price of natural gas (which increased to above $6/MMBtu) combined with low electricity prices caused the gas turbine combined cycle plants to be operated at only 30% average capacity (50-60% for the SGT6-6000G). The increase of natural gas prices caused combined cycle plants to move lower in the dispatch order and therefore into a cycling duty mode. In this environment, the amount of time the merchant plants could operate profitably was reduced. In response to the changed market requirement for more cyclic operation and to further enhance its competitive advantage, design changes and improvements were incorporated into the SGT66000G engine. Other enhancements are planned for the future to allow even more operating flexibility. Customer feedback through user groups, direct feedback and the Diagnostics Centers have allowed Siemens to focus on development programs which are directly aligned with customer requirements. SGT6-6000G was readily adaptable to cyclic/flexible operation due to its inherent design 2 features and this capability was further improved by enhancements incorporated over the last several years. The following enhancements were developed in cooperation with SGT6-6000G operators in order to address their short and long term needs: • Design improvements were incorporated into the combustor basket to reduce emissions, improve reliability and increase the time between inspection intervals. • The steam cooled transition was redesigned to reduce metal temperatures and extend inspection intervals based on both hours and starts. • Compressor and turbine sealing were improved, thus improving performance and reducing emissions. • Cooling designs on the first four rows of turbine airfoils were optimized. • The rotor cooling air temperature was optimized to enhance operational flexibility. • Enhanced turbine disk material, which is utilized in the existing V-fleet, was instituted. • The exhaust system was redesigned to improve performance and service life. • Trip Factor number (number of equivalent starts for each engine trip) was reduced. • Starting was changed from mechanical motor start to static start utilizing a Static Frequency Converter (SFC) to improve starting reliability and reduce capital cost. • Gas turbine and plant controls were optimized and simplified to improve the engine’s operational flexibility and starting reliability. Operating Experience The prototype SGT6-6000G engine was started up in April, 1999. This plant was initially in a simple cycle configuration with the once-through auxiliary boiler producing transition cooling steam. After the extensive verification test program was completed, the plant went into commercial operation in March, 2001. The plant was converted to combined cycle configuration by the end of 2001. The first SGT6-6000G in CC application achieved commercial operation in April, 2001, at the PG&E Millennium Plant in Charlton, Massachusetts. References 10 to 13 provide additional information on SGT6-6000G operational experience. Figure 1. SGT6-6000G Fleet Operating Profile in 2004 SGT6-6000G engines have operated in different modes, from peaking duty, through cycling/load following, to base load duty (see Figure 1). They demonstrated excellent operational and starting reliability shortly after their introduction, over a wide ambient temperature range, on both gas 3 and oil fuels. At one site, during the second commercial operation year, the gas turbine operated at 10% capacity and had 123 successful starts with a 95% starting reliability (see Reference 13). The SGT6-6000G functioned satisfactorily in this highly cyclic operational mode. Many financial and insurance institutions considered 8,000 operating hours on a new frame a benchmark that defines the product as “mature”. The operating fleet has reached and surpassed this milestone, since currently 10 engines have accumulated more than 8,000 hours. By the middle of 2005, the lead unit has been in commercial operation for more than 22,000 hours, and the total fleet has accumulated more than 155,000 operating hours. The fleet operating hours are being accumulated at a fast rate by the 20 units in commercial operation and will accelerate with new units coming on line in the near future. Figures 2 and 3 show the increase with time of the total fleet operating hours and the number of operating units. 0 5000